The pineal gland hormone melatonin is a potent inhibitor of tumorigenesis in experimental models of breast cancer while, in humans higher and lower nocturnal melatonin blood levels are associated with lower and higher breast cancer risk, respectively. Therefore, the long-term objective of the proposed translational research project is to gain a new understanding of the role of melatonin, derived from dietary supplements, as a potentially new complementary medical strategy for preventing the growth of human breast cancer. The hypothesis to be tested is that human females who have ingested melatonin from dietary supplements will have levels of melatonin in their blood which, when perfused through human breast cancer xenografts in female nude rats, will play a significant role in the treatment and prevention of the growth and metabolic activity of those tumors. This occurs through a melatonin receptor-mediated mechanism involving the suppression of 1) cAMP-dependent tumor linoleic acid (LA) uptake, 2) 13-hydroxyoctadecadienoic acid (13-HODE) formation, leading to a down-regulation of the 3) MEK/ERK1/2 and Akt/mTOR cancer growth/survival signaling pathways. Our experimental approach involves a novel model system in which human breast cancer xenografts, growing in female nude rats, are directly perfused in situ with whole blood collected from human female subjects prior to and following the ingestion of a dietary melatonin supplement. The first aim is to determine the dose-response effects of melatonin derived from dietary supplements on tumor proliferative activity, LA uptake and 13-HODE formation, and signal transduction activity in tissue- isolated ER+ and ER- MCF-7 human breast cancer xenografts directly perfused in situ with melatonin-rich whole blood from pre- and postmenopausal female volunteers following the oral intake of a commercially available melatonin supplement. The second specific aim is to test the effects of a specific melatonin receptor blocker on the ability of melatonin-rich whole blood, from human volunteers following the oral intake of a melatonin supplement, to suppress tumor proliferative activity and LA uptake and metabolism, and signal transduction activity in ER+ and ER- human breast cancer xenografts during perfusion in situ. In both aims, the effects of melatonin from dietary supplements will be determined on tumor cAMP levels and the activation of the MEK/ERK1/2 and Akt/mTOR proliferation/survival signaling pathways. The knowledge obtained from this innovative, translational approach will provide a rational biological basis for the design of the first large-scale clinical breast cancer treatment and/or prevention trials using commercially available melatonin supplementation as a new complementary medical strategy. PUBLIC HEALTH RELEVANCE: The recent identification of a significantly increased risk of breast cancer in women who work night shifts together with increased public awareness and widespread use of over-the-counter melatonin supplements, makes melatonin supplementation an important public health issue in breast cancer risk, prevention and treatment. The information gained from our novel research approach will provide a critical step for the future design of the first clinical trial of melatonin supplementation for breast cancer treatment and prevention particularly in high risk populations such as night shift workers.